1. FIELD OF THE INVENTION
This invention relates to a reflection type photoelectric switch. More particularly, this invention relates to a reflection type photoelectric switch adated to detect markings on a sheet or the like which is driven by a roller device, for instance.
2. DESCRIPTION OF THE PRIOR ART
In the conventional photoelectric switch of the reflection type for detection of markings on a sheet, a condenser lens is used to focus a light beam from a light emitting element in a predetermined position on the sheet and a marking on the sheet is detected from a change in the level of reflected light. Since the reflected light level is changed by delicate displacement of the sheet in the direction of an optical axis, such as conventional reflection type photoelectrical switch is liable to detect a marking in a false position. To overcome this drawback, there has been proposed a mark sensor wherein two distinct light beams are projected to different positions on the sheet and a marking on the sheet is detected in the differences from the respective reflected light levels. However, such a technique has the disadvantage that the construction of the optical system is complicated.